Adapter for low-frequency amplifiers



March 26, 1940. p VAN m 2,194,534

ADAPTER FOR LOW-FREQUENCY AMPLIFIERS Filed Nov. 13, 1937 //V l/ E N TOR P/FTFR FQFDER/K I44 510/! ATTORNEY Patented Mar. 26, 1940 UNlTEDi. JST'ATES' PATENT OFFICE 2,194,534 I ADAPTER FOR LOW-FREQUENCY AMPLIFIERS Pieter Frederik van Eldik, Eindhoven, Nethera lands, assignor, by mesne assignments, to Radio [Corporation of America, New York, .N. 1 a

corporation of Delaware r Application November 13, 1937, Serial No. 174,315

. In Germany December 28, 1936 2 Claims.

This invention relates to a pro-connected apparatus for low frequency amplifiers by means of for each of the devices which produce or conduct audio-frequency oscillations. and to which the amplifier is to be connected. These transformers are given such a size thatcorrect adaptation is ensured for all devices.

The invention consists in providing a low frequency transformer designed for the adaptation and common to all devices, whereas impedances, preferably resistances, are provided for adaptation of the said devices or part thereof to the I transformer. Thus the advantage is obtained that the device is extremely simple and cheap.

The drawing represents one form of construction of an apparatus according to the invention. The apparatus comprises a low frequency transformer l whose secondary winding can be con-.

nected, through a change-over switch described hereinafter, to the terminals 2 for connecting the amplifier. The secondary winding is bridged by a potentiometer-resistance 3 part of which can be short-circuited by means of a contact 4 and which serves for the volume control. In addition the apparatus comprises terminal pairs 5, 6, l and 9 for connecting the different devices producing or conducting oscillations. The terminals 5 are provided for connecting a radio set. For achieving correct adaptation these terminals are bridged by a resistance In and the voltage to be amplified is derived from a small part of this resistance. This voltage is supplied to the primary transformer winding through the mutually coupled switches l2 and I3.

The terminals 6 are provided for connecting a transmission line, for instance, a telephone line. In this case, also, only a small part of theavailable voltage is supplied to the primary transformer winding. For this purpose a resistance I I of which points symmetrically placed with respect to the middle are connected to the switch contacts in parallel with the said terminals. If desired, the middle of the resistance ll may be earthed. When the switch l2, l3 bears on the said contacts connected to the resistance II the device is adapted for the connection of a telephone line. In this manner the telephone line remains entirely symmetrical in electrical respect.

A third pair of terminals 1 is provided for connecting a microphone. It is presumed that this microphone is adapted without using separate adapting resistances. The low frequency transformer may be given such a size that suitable adaptation is obtained for this case without using separate impedances. The feed current for the microphone is derived from the terminals 8 one of which is connected to a third switch lever l4 mechanically coupled to the two firstmentioned switch levers l2 and I3. It will be obviousthat when the switch leversbear onthe fourth contact of each series of contacts the apparatus is adapted for the reproduction of microphone currents.

In addition thesaid switch has a position of rest wherein the primary transformer winding is connected to none of thepairs of terminals. This applies when the switch levers bear on the second contact of each series.

The above devices permit several apparatus to be connected to a low frequency amplifier by means of a single low frequency transformer. The fifth pair of terminals 9 serves for connecting 'a pick-up, in which case a low frequency transformer can be dispensed with. Consequently these terminals are directly connected to the terminals 2 through a switch 15 which has also five positions and is coupled to the said switches. In this case volume control is not necessary since this is in general directly provided on the pick-up. In the position of rest of the switch the terminals 2 are short-circuited.

What I claim is:

1. In low frequency amplifying apparatus, a plurality of individual lines each including means for connection thereof to a different source of energy to be amplified, a pair of terminals adapted to be connected to the input of an amplifier, a coupling transformer including a. primary winding and a secondary winding, said secondary winding being connected across said pair of terminals, individual impedances connected across each of said lines, selective switching means interposed between the primary winding and said lines for connecting the primary winding across portions of any one of said impedances, said portions being arranged so that impedance match is obtained "between each of the lines and the transformer, said switching means comprising a plurality of uni-controlled switches each havin a plurality of contact points determined by the number of individual lines arranged to be connected to the transformer primary and a blank contact position wherein the primary of the 2. In low frequency amplifying apparatus, a plurality of individual lines each including means for connection thereof to a different source of energy to be amplified, a pair of terminals adapted to be connected to the input of an amplifier, a coupling transformer including a primary winding and a secondary winding, said secondary m winding being connected across saidpair'of terminals, individual impedances connected across each of said lines, selective switching means interposed between the primary Winding and said lines for connecting the primary winding across 15 portions of any one of said impedances, said portions being arranged so that impedance match is obtained between each ofthe lines and the transformer, said switching means comprising a plurality of uni-control switches each having a plurality of contact points determined by the number of individual lines arranged to be con? I nected to the transformer primary, a blank contact position wherein the primary of the trans former winding is left open and the secondary of the transformer is short-circuited, and an addi-' nected directly across the first named pair of terminals. PIE'IER FREDERIK VAN ELDIK.

, 10 tional position wherein one of said lines is con- 

